Many household and industrial machines and devices are powered by a direct current (DC) power source that has been rectified from alternating current (AC) power provided by the AC mains. The AC-to-DC rectification is typically accomplished using a bridge rectifier 104 (or “diode bridge”) comprised of four diodes 102-1, 102-2, 102-3 102-4 configured as shown in FIG. 1. The bridge rectifier 104 converts the positive and negative half cycles of the AC input voltage Vin to a full-wave-rectified waveform of constant polarity. (See FIGS. 2A and 2B). To produce the desired steady DC output voltage Vout across a load 108, the rectified waveform is filtered by a smoothing circuit, which in its simplest form comprises a smoothing capacitor 106 coupled to the output of the bridge rectifier 104. The smoothing capacitor 106 functions to maintain the DC output voltage Vout near the peak voltage Vpeak during the low portions of the AC input voltage Vin, as shown in FIG. 2C. Some amount of AC ripple is superimposed on the DC output Vout, even following filtering by the smoothing capacitor 106. The ripple may or may not be tolerable, depending on the application. In applications where it is not tolerable, additional filtering can be employed to reduce it to an acceptable level.
The AC/DC converter 100 in FIG. 1 generates a DC output voltage Vout near the peak voltage Vpeak of the AC input voltage Vin (see FIG. 2C). However, many applications require a much lower voltage. For example, many machines and devices require a DC voltage of 12 volts DC or less but the peak voltage Vpeak of the center-tapped 120 volts RMS (root mean square) residential mains is near 170 V. To lower the DC voltage to the required level, a step-down transformer or DC-DC converter 302 (i.e., “buck converter”) is used. FIG. 3 illustrates use of a DC-DC converter 302. The DC-DC converter 302 comprises a switch (typically a metal-oxide-semiconductor field effect transistor (MOSFET)) 304, a diode (or, alternatively, a second MOSFET) 306, an inductor 308, a filter capacitor 310, and a pulse-width modulator (PWM) control 312. The PWM control 312 controls the opening and closing of the switch 304 at a fixed frequency f that is much higher than the 60 Hz line frequency (typically greater than 1 kHz). When the switch 304 is turned on, current flows through it, the inductor 308, and then into the filter capacitor 310 and the load 108. The increasing current causes the magnetic field of the inductor 308 to build up and energy to be stored in the inductor's magnetic field. When the switch 304 is turned off, the voltage drop across the inductor 308 quickly reverses polarity and the energy stored by the inductor 308 is used as a current source for the load 108. The DC output voltage Vout is determined by the proportion of time the switch 304 is on (tON) in each period T, where T=1/f. More specifically, Vout=DVin(dc), where D=tON/T is known as the “duty cycle” and Vin(dc) is the source DC input voltage provided at the output of the bridge rectifier 104. The PWM control 312 is configured in a feedback path, allowing it to regulate the DC output voltage Vout by modulating the duty cycle D.
Although the AC/DC converter 300 in FIG. 3 addresses the inability of the AC/DC converter 100 in FIG. 1 to step down the DC voltage to a lower DC voltage, it does not address another well-known problem of conventional AC/DC converters—low power factor. The power factor of an AC/DC converter is a dimensionless number between 0 and 1 indicating how effectively real power from an AC power source is transferred to a load. An AC/DC converter with a low power factor draws more current from the mains than one having a high power factor for the same amount of useful power transferred. A low power factor can result due to the input voltage Vin being out of phase with the input current Iin or by action of a nonlinear load distorting the shape of the input current Iin. The latter situation arises in non-power-factor-corrected AC/DC converters, such as those described in FIGS. 1 and 3, which as described above use a diode bridge 104. The filter capacitor 106 of the AC/DC converter 100 in FIG. 1 (and, similarly, the filter capacitor 310 of the AC/DC converter 300 in FIG. 3) remains charged near the peak voltage Vpeak for most of the time. This means that the instantaneous AC line voltage Vin is below the filter capacitor 106 voltage for most of the time. The diodes 102-1, 102-2, 102-3 102-4 of the bridge rectifier 104 therefore conduct only for a small portion of each AC half-cycle, resulting in the input current Iin drawn from the mains being a series of narrow pulses, as illustrated in FIG. 4. Note that although the input current Iin is in phase with the AC input voltage Vin, it is distorted and, therefore, rich in harmonics of the line frequency. The harmonics lower the power factor, resulting in reduced conversion efficiency and undesirable heating in the AC mains generator and distribution systems. The harmonics also create noise that can interfere with the performance of other electronic equipment.
To reduce harmonics and increase the power factor, conventional AC/DC converters are often equipped with a power factor correction (PFC) pre-regulator. The PFC pre-regulator can be formed in various ways. One approach employs a PFC boost converter 502 coupled between the bridge rectifier 104 and the DC-DC converter 302, as shown in the power-factor-corrected AC/DC converter 500 in FIG. 5. The PFC boost converter 502 comprises an inductor 504, switch 506, diode 508, output capacitor 510 and a PFC control 512. The PFC control 512 controls the on and off state of the switch 506. When the switch 506 is switched on, current from the mains flows through the inductor 504, causing energy to build up and be stored in the inductor's magnetic field. During this time, current to the DC-DC converter 302 and load 108 is supplied by the charge in the capacitor 510. When the switch 506 is turned off, the voltage across the inductor 504 quickly reverses polarity to oppose any drop in current, and current flows through the inductor 504, the diode 508 and to the DC-DC converter 302, recharging the capacitor 510 as well. With the polarity reversed, the voltage across the inductor 504 adds to the source input DC voltage, thereby boosting the input DC voltage. The PFC boost converter 502 output voltage is dependent on the duty cycle D of the on-off switch control signal provided by the PFC control circuit 512. More specifically, the PFC boost converter 502 output voltage is proportional to 1/(1−D), where D is the duty cycle and (1−D) is the proportion of the switching cycle T (i.e., commutation period) that switch 506 is off. In addition to setting the duty cycle D, the PFC control 512 forces the DC-DC converter 302 and load 108 to draw current that on average follows the sinusoidal shape of the AC input voltage Vin, thereby reducing harmonics and increasing the power factor of the AC/DC converter 500.
The power-factor-corrected AC/DC converter 500 is suitable for many applications. However, it has a number of drawbacks. First, the AC/DC converter is less efficient than desired, particularly since the AC-to-DC power conversion requires two stages—the PFC boost converter 502 front end and the DC-DC converter 302 final stage. Second, the converter 500 has a large parts count, including parts necessary to implement the two control circuits (PFC control 512 and PWM control 312), which increases design complexity and cost, and makes the converter 500 more susceptible to failure. Third, the PFC boost converter 502 generates very high voltages, which stress the converter's parts and raise safety concerns.
It would be desirable, therefore, to have AC/DC conversion methods and apparatus that are efficient at converting AC to DC, avoid power factor degradation attributable to using a bridge rectifier, do not require voltage boosters to counteract power factor degradation, and do not have a large parts count.